Pure Substances, Elements, and Atomic Structure Basics

Pure Substances and Matter Classification

A homogeneous material made up of one kind of substance is a pure substance.

Every pure substance differs from others by its characteristic properties, which allow recognition.

Chemical Compounds and Elements

A chemical compound is a pure substance that can decompose, sometimes with difficulty, into simpler substances.

An element is a pure substance that cannot be broken down into more simple substances. Elements combine to create new pure substances, which are compounds.

The two most abundant elements in Earth's crust are oxygen and silicon.

Properties of Metals and Non-metals

• Metals are generally good conductors of heat and electricity.
• Non-metals (except for graphite) are poor conductors of heat and electricity.

Examples of Elements

Some metals: Aluminum (Al), Silver (Ag), Barium (Ba), Calcium (Ca), Copper (Cu), Chromium (Cr), Tin (Sn), Iron (Fe), Magnesium (Mg), Mercury (Hg), Nickel (Ni), Gold (Au), Platinum (Pt), Lead (Pb), Sodium (Na), Titanium (Ti), Zinc (Zn).

Some non-metals: Argon (Ar), Bromine (Br), Carbon (C), Chlorine (Cl), Fluorine (F), Phosphorus (P), Helium (He), Hydrogen (H), Iodine (I), Neon (Ne), Nitrogen (N), Oxygen (O), Silicon (Si), Sulfur (S).

Atomic Structure Fundamentals

Matter is discontinuous, constituted from tiny particles called atoms (volumes). An atom is the smallest part of an element that can exist. The atoms of one element are different from atoms of another element.

Molecules

Two or more atoms can join with each other, forming a group called a molecule. If the atoms united to form a molecule are of the same element, we have a molecule of an element.

If the atoms united to form the molecules are of different elements, it forms the molecule of a compound, called a molecular compound.

The molecular formula shows the number and kind of atoms that form a molecule.

Subatomic Particles

Atoms are formed by smaller particles also called subatomic particles. An atom consists of a nucleus (the central part), and an envelope or electronic layer.

Particle Charges

• The protons carry a positive electric charge (+) and are represented with the letter P.
• The neutrons have no electric charge, and their mass is approximately equal to the proton's. They are represented with the letter N.
• Electrons have a negative electrical charge (-), like the proton but opposite in sign. They are represented with the letter e^-.

An atom is electrically neutral; it has no excess of positive or negative charges, meaning the number of protons in the nucleus is equal to the number of bound electrons. Electrons have a much smaller mass and move around the nucleus at high speed. The nucleus of an atom occupies a very small volume when compared to the total volume of the atom.

Ions and Compound Types

If an atom loses one or more electrons, it becomes an ion with a positive electric charge, called a cation.

If an atom gains one or more electrons, it becomes a negatively charged ion, called anion.

The chemical compounds formed by ions, such as common salt, are called ionic compounds.

The compounds formed by molecules, such as water, are called molecular compounds.